WPF Rounded Corners background bleeding through - wpf

Question

I'm making my first foray into WPF - I have a simple form with a popup defined for inline help. I'm using rounded corners, and for some reason a black background is bleeding through around the corners. I don't understand which element is causing the problem.
alt text http://www.awbrey.net/rounded.jpg
I assume it's something blindingly obvious which I'm just not seeing. Here's the XAML I'm using:
<Window x:Class="Consent.Client.SubjectNumberEntry"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" FontSize="24"
Title="SubjectNumberEntry" WindowStyle="None" WindowState="Maximized"
xmlns:h="clr-namespace:Consent.Client" KeyDown="windowOuter_KeyDown" Background="White" Name="windowOuter" AllowsTransparency="true" Loaded="Window_Loaded">
<StackPanel Height="400" DockPanel.Dock="Top" HorizontalAlignment="Center" VerticalAlignment="Center" Margin="10">
<StackPanel Height="60" Orientation="Horizontal" VerticalAlignment="Center">
<TextBox Name="txtSubjectNumber" Margin="10" Width="400" KeyDown="txtSubjectNumber_KeyDown" h:HelpProvider.HelpString="Enter the subject identifier, or scan their wristband">
<TextBox.ToolTip>This is a textbox</TextBox.ToolTip>
</TextBox>
<Button Name="btnEnter" Margin="10" Width="100" Click="btnEnter_Click">Enter</Button>
<Button Width="50" Name="btnHelp" Margin="10" Click="btnHelp_Click">?</Button>
<Button Width="50" Name="btnExit" Margin="10" Click="btnExit_Click">Exit</Button>
</StackPanel>
<Label Name="lblValue" Margin="10"></Label>
<Popup Placement="Bottom" HorizontalAlignment="Center" VerticalOffset="10" MouseDown="popHelp_MouseDown" PopupAnimation="Fade" Name="popHelp" PlacementTarget="{Binding ElementName=txtSubjectNumber}">
<Border Padding="10" Margin="10" BorderBrush="CornflowerBlue" BorderThickness="1" CornerRadius="10" Background="CornflowerBlue">
<TextBlock FontSize="12" Background="CornflowerBlue">This is the content of the help box.</TextBlock>
</Border>
</Popup>
</StackPanel>
</Window>

Answer 1

I think it is the Popup that is causing the problem. Try setting AllowsTransparency to True on the popup.
Popup.AllowsTransparency
When set to False, any transparent colors are "merged" with black.

Answer 2

You can also wrap the popup in a border that has rounded corners. This is useful if you can't change the AllowsTransparency of the popup.
Something like this:
<Border CornerRadius="10">
<Popup Placement="Bottom" HorizontalAlignment="Center" VerticalOffset="10" MouseDown="popHelp_MouseDown" PopupAnimation="Fade" Name="popHelp" PlacementTarget="{Binding ElementName=txtSubjectNumber}">
<Border Padding="10" Margin="10" BorderBrush="CornflowerBlue" BorderThickness="1" CornerRadius="10" Background="CornflowerBlue">
<TextBlock FontSize="12" Background="CornflowerBlue">This is the content of the help box.</TextBlock>
</Border>
</Popup>
</Border>